Methods using a simulated moving bed separation system have been proposed for separating a desired component from a multicomponent mixture (see, for example, JP-B-42-15681).
In the simulated moving bed separation system proposed in the above-mentioned patent, there are provided four work zones packed with a solid adsorbent and connected to each other in a line, with the outlet of the fourth work zone being connected to the inlet of the first work zone so that a fluid can circulate through these four work zones. Also, a port for feeding a multi-component feedstock mixture to be treated into said work zones, a port for drawing out a weakly adsorbable component (raffinate) from said work zones, a port for supplying a desorbing liquid into the work zones and a port for drawing out the desorbing liquids and a strongly adsorbable component (extract from the work zones are provided so that the positions of the ports will advance simultaneously at given time intervals.)
The solid adsorbent packed in said work zones is a fine powder with a particle size of usually 15 mm or greater (see page 11, right column, lines 6-10 of JP-B-42-15681). Solid adsorbents with such a large particle size enable high-rate introduction of feedstock mixture and desorbing liquids. However, in order to accomplish separation at an even higher efficiency and higher rate by using the above simulated moving bed separation system, it is necessary to make smaller the particle size of the solid adsorbent. This, however, involves the difficult problems. When a solid adsorbent with a smaller particle size than those used hitherto is packed in the work zones and a feedstock mixture and a desorbing liquids are introduced into these work zones at high speed as in the prior art, there tends to occur a pressure loss in the work zones and back flow of the circulating fluid takes place in the work zones. Back flow of the circulating fluid causes a serious reduction of separating efficiency. Thus, when it is tried to improve the separating efficiency by reducing the particle size of the solid adsorbent, it is necessitated to drop the feed rate of the feedstock fluid into the work zones, and such drop of the feed rate decreases the separating efficiency to an intolerable level for industrial practice.
Also, in case the packing used in the simulated moving bed is already small in particle size, when the feedstock fluid charging rate is increased for further improving the separating efficiency, there takes place back flow of the circulating fluid as mentioned above to lower the separating efficiency.
Actually, many attempts have been made in recent years for separating optical isomers by using a simulated moving bed system of the described type. In these operations, and optical resolving agent with a very small particle size, which is in the order of one to several ten microns, is packed in the beds, and a feedstock mixture and a desorbing liquids are introduced at high rates in between the adjoining unit packed beds. In this case, the pressure loss by the packing is very great and the pressure at the outlet of a unit packed bed would become lower than the pressure of the desorbing liquid or feedstock fluid, causing such desorbing liquid or feedstock fluid to flow in the direction opposite to the flow of the circulating fluid.
The conventional simulated moving bed separation system, when described in other terms, comprises a plurality of unit packed beds connected in series to each other, in which a solution containing a feedstock mixture and a desorbing liquid are introduced into the packed bed containing an optical resolution packing therein and having front and rear ends thereof connected to each other endlessly via a fluid passage to circulate a fluid unidirectionally while at the same time drawing out a solution containing a separated component and a solution containing another component from the packed bed, wherein a port for introducing a desorbing liquids, a port for drawing out a solution containing a strongly adsorbable substance (extract), a port for introducing a solution containing a feedstock mixture, and a port for drawing out a solution containing a weakly adsorbable substance (raffinate) are arranged in the packed bed in this order along the direction of fluid flow and the positions of these ports are successively moved in the direction of fluid flow in the packed bed intermittently.
Also, in the conventional simulated moving bed separation system, in order to successively move said introducing a draw-out ports intermittently, there are provided four rotary valves, one for introducing a solution containing a feedstock mixture, on for introducing a desorbing liquids, one for drawing out an extract and one for drawing out a raffinate. Further, a circulation pump is disposed between a specific unit packed bed and another unit packed bed positioned downstream of said specific unit packed bed for circulating a fluid when moving said introducing and draw-out ports successively by switching said four rotary valves.
In operation of the simulated moving bed separation system, when the positions of said desorbing liquids introducing port, feedstock fluid introducing port, extract draw-out port and raffinate draw-out port are shifted by change-over operations of said rotary valves through a distance corresponding to one unit packed bed, the works to be performed in the respective unit packed beds, namely adsorption, concentration, desorbing and recovering of the desorbing liquids, are switched correspondingly. In each of said working steps, there is a setting flow rate which has been determined for accomplishing the desired separation.
Therefore, as the works to be performed in the respective unit packed beds are shifted by the rotary valve change-over operation, the flow rate of the circulation pump must be changed in accordance with the work to be performed.
It is essential that there is no difference between the circulation flow rate in the line preceding the circulation pump and the flow rate in the line succeeding said pump and between the feed rate into the circulation pump and the discharge rate from said pump. Therefore, the flow rate of the circulation pump must be adjusted stepwise synchronously with all of the other rotary valves upon every change-over operation of a rotary valve.
Thus, when the circulation pump was set between a specific unit packed bed and the adjoining unit packed bed on the downstream side, it was necessary to perform the troublesome operation of stepwise increasing or decreasing the fluid flow rate in the circulation pump upon every shifting of said introducing and draw-out ports in the direction of fluid flow.
The present invention has been made in view of the above circumstances.
Accordingly, the first object of the present invention is to provide a high-efficiency simulated moving bed separation system which is capable of separating a desired component from a feedstock mixture at high speed and high efficiency even when a feedstock fluid and a desorbing liquid are introduced at a high rate into an inlet port provided between a unit packed bed and the succeeding unit packed bed, each bed filed with a packing having a very small particle size.
The second object of the present invention is to provide a rotary valve-incorporated simulated moving bed separation system which has excellent operating performance and is suited for separating the individual components from a mixture of similar compounds and which can unnecessitate the conventional troublesome operation of stepwise adjusting the flow rate of the circulation pump upon every positional shifting of the introducing and draw-out ports.